Mussel culture is done under sea water, generally. Culture ropes are hung underwater from various structures such as a raft or a long horizontal rope referred to in the field as a “mainline” or “back line”. Buoyancy devices are tied to the mainline with the culture ropes hanging below the mainline.
In nature, wild mussel spat or “seeds” settle on the culture ropes at the end of the larvae life. The culture ropes preferably have a filamentous surface, simulating natural settling environments such as algal mats. In certain applications, however, mussel culture ropes are made of used ropes discarded by other fisheries, for example.
Subsequently to spat collection, the mussels are either left to grow on the culture ropes; transferred to tubular sock-like netting, or left on the ropes and encapsulated into tubular sock-like netting. The netting is often made of cotton which eventually dissolves to facilitate harvest.
Amongst the factors affecting crop yields, there are of course; water quality, and the abundance of food at a particular site. There are, however, three additional factors that can bring about great losses despite being at a perfect culture site. These factors are: limited attachment surface area provided by the culture ropes; predation of juvenile mussels by finfish and diving ducks; and the rubbing of culture ropes against each other in rough seas.
Referring to the first disadvantage, population density on a mussel collector undergoes a self-thinning process as mussels grow. Some fall off the culture ropes because of a lack of substrate surface area to attach themselves to. Mussels falling off the ropes are destroyed by crabs, starfish and other animals living in the sea floor habitat.
There is more to consider in reference with predation by other animals. Some duck species can dive to 10 meter (33 feet) underwater for over one minute at a time. Although ducks prefer small mussels under 30 mm (1.2 inch) in diameter, they can eat larger mussels, swallowing shell and all and crushing the shells in their gizzard (muscular stomach). Mussel predation is a serious problem as in many cases mussel population on culture ropes can be reduced to the point where operations are no longer profitable. Ideally, mussel spat need to be protected from sea ducks until they can reach a size of about 4 cm (1.6 inch) in diameter.
In rough seas, suspended culture ropes are sometime brought closer to each other than their initial setting. Some mussel spat attach themselves to these juxtaposed ropes. Plain ropes offer no hollow, crevice or cavity to shelter and to protect cultured mussels from being scraped off the ropes during the rubbing of these ropes against each other. Therefore, some culture ropes can be wiped clean of mussel spat in a single storm.
The following documents represent a good inventory of the mussel culture methods and installations included in the prior art.
A New Rope for Mussel Farming: published in Aquaculture, 8 (1976) pp. 383-388, by Elsevier Scientific Publishing Company, Amsterdam, authored by Philip Tortell on Apr. 22, 1976. This document describes a fibrous rope having tufts of fibers spaced there along to facilitate collection and retention of mussel spat to the rope.
U.S. Pat. No. 3,678,900 issued to C. M. Kolesa on Jul. 25, 1972. This document discloses a system for harvesting shellfish using culture chains that are laid directly on the sea floor.
U.S. Pat. No. 7,341,021 issued to A. Q. Cortinas, et al., on Mar. 11, 2008. This document illustrates mussel culture ropes suspended vertically underwater to parallel cables.
CA Patent 2,178,203 issued to J. D. Saxby et al., on Jun. 15, 1995. This document discloses a method for cultivating mollusks using a netting bag suspended horizontally under water.
CA Patent Application 2,420,918, filed by Gilles Gagnon on Mar. 5, 2003. This document discloses a disc-like device for attachment to a mussel culture rope to prevent slipping of the mussels as they grow and overcrowd the ropes.
CA Patent 2,332,086 issued to Ian W. Jefferds on Aug. 19, 2003, This document discloses mussel culture ropes suspended underwater to a floating raft. Disc-like supports are mounted at intervals to each rope to prevent slipping of the mussels as they grow and the culture rope becomes over populated.
CA Patent Application 2,535,821 filed by P. Kvietelaitis, on Sep. 10, 2004. This document discloses culture ropes suspended underwater to a mainline supported by buoys. The culture ropes are encapsulated in netting socks.
CA Patent Application 2,556,556 filed by P. Kvietelaitis on Feb. 18, 2005. This document discloses an apparatus and a method for seeding and encapsulating a culture rope with young mussels.
Although the method of encapsulating a culture rope in a protective netting sleeve provides protection to growing mussels against predators and against slipping, the netting material represents an impediment to mussel spat retrieval prior to sleeving and may interfere with normal growth. Therefore, there is still a need in the aquaculture industry for a mussel culture installation that can provide both an efficient collection of mussel spat at the beginning of the culture cycle and a good protection of young mussels from predators and from shrinking attachment surface area.